Means for damping voltage oscillations in solid-state switching circuits

ABSTRACT

A thyristor is serially joined to an inductor and is shunted by a snubber circuit comprising a resistor in series with a capacitor. When subjected to a stepped change in voltage, the series combination of the inductor and the snubber circuit may become oscillatory. To prevent the voltage across the thyristor from undesirably overshooting, a tuned damping circuit is connected in parallel with the inductor, or alternatively in parallel with the snubber circuit.

I United States Patent [151 3,675,110

Kelley, Jr. 51 July 4, 1972 s41 MEANS FOR DAMPING VOLTAGE 2,730,667l/l956 Uhlmann ..32l/11 x ()SCILLATIQNS 1 SOLID.STATE 3,253,138 3732(7)guff ..32l/45 C ,0 reening eta ..321/1 1 X SWITCHING CIRCUITS 3,532,90110/1970 Hylten-Cavallius et al.... ..32l/ll [72] Inventor: Fred W.Kelley, Jr., Media, Pa. 3,569,819 3/1971 Martzloff et al. ..321/l2 [73]Assign: General Electric Company Primary Examiner-William M. Shoop, Jr.2 Filed; 27 1 Attorney-J. Wesley Haubner, Albert S. Richardson, Jr.,

Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman [21] Appl.No.: 110,165

, [57] ABSTRACT [52] US. Cl. ..32l/l4, 321 /5, 321 /27 R, A thyristor isserially joined to an inductor and is shunted by a 321/45 C snubbercircuit comprising a resistor in series with a capacitor. 51 1nt.Cl...H02m l ls When subjected to a pp change in g the series 53 Field ofSearch 321 5 11 12 14 27 45 combination of the inductor and the snubbercircuit may become oscillatory. To prevent the voltage across thethyristor [56] (defences Cited from undesirably overshooting, a tuneddamping circuit is connected in parallel with the inductor, oralternatively in UNITED STATES PATENTS parallel with the snubbercircuit.

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P'ATENTEDJUL 4 I972 ATTORNEY MEANS FOR DAMPING VOLTAGE OSCILLATIONS INSOLID-STATE SWITCHING CIRCUITS This invention relates to solid-stateswitching circuits, and, more particularly, it relates to electric powerconversion apparatus in which said circuits are used.

There are many known power conversion circuits for changing the form ofelectric power from direct current to alternating current, fromalternating current to unipolarity or bipolarity direct current, or frompolyphase alternating current of fundamental frequency to single-phaseor polyphase alternating current of a different frequency. Suchapparatus are popularly referred to as inverters, rectifiers, reversingrectifiers, cycloconverters, or direct frequency changers (ormultipliers). In each case the conversion is accomplished byappropriately controlling a plurality of periodically conducting,sequentially fired electric valves that are interconnected in a bridgeconfiguration between d-c and a-c temiinals, the latter being adapted tobe connected to a system of alternating voltage with which the valvefirings are synchronized.

in modem practice each valve typically comprises one or more solid-stategate-controlled switching components known as semiconductor controlledrectifiers or thyristors. A conventional thyristor is physicallycharacterized by a body of monocrystalline silicon between a pair ofmain current-carrying metallic electrodes known as the anode and thecathode, respectively. The silicon body may comprise, for example, athin, broad area disc-like wafer having four layers of altemately P andN type conductivities, whereby three back-to-back PN (rectifying)junctions are formed between the main electrodes. Usually the wafer ismechanically sealed in an insulating housing and is electricallyconnected in an external power circuit by way of its anode and cathode,and the device is equipped with suitable gating means for initiatingconduction between these main electrodes on receipt of a predeterminedcontrol or trigger signal.

In operation, each valve of the converter has a nonconducting orblocking statejin which it presents very high impedance to the flow ofcurrent, and a conducting or tumed-on state in which it freely conductsforward current between its main electrodes. It can be switched abruptlyfrom the former state to the latter by applying a trigger signal to itsgate when its main electrodes are forward biased (anode voltage positivewith respect to cathode). This switching or firing" action is ordinarilycontrolled by associated regulating and control circuits which supply atrain of properly timed and dimensioned gate pulses to the valve. Inmany converters the moment of time at which the valve is fired isexpressed in electrical degrees (known as the firing angle) measuredfrom the cyclically recurring instant at which its anode voltage firstbecomes positive with respect to cathode, and the magnitude of theoutput voltage of the converter can be varied by retarding or advancingthe firing angle as desired. Once turned on, a valve will continueconducting until forward" current is subsequently reduced below a givenholding level by the action of the external circuit in which the valveis conneCted. This tumoff process is referred to generally ascommutation." A variety of line and load voltage commutated convertersand forced commutated inverters are well known in the art.

During commutation, current in a solid-state valve will actuallydecrease to zero and then reverse directions for a brief moment of timeuntil the rectifying junctions inside the thyristors forming that valvehave recovered their ability to withstand reverse voltage, whereupon thevalve suddenly regains its reverse blocking state. Since there is alwayssome circuit inductance in series with the valve, any sudden cessationof current can lead to undesirable voltage transients across the valve.To suppress such transients, it is common practice to shunt the valvewith a so-called snubber" circuit comprising a capacitor and a resistorin series.

Snubber circuits are also beneficial in certain inverter applicationswhere a valve is subjected to a very high time rate of rise of forwardanode voltage at the conclusion of an interval of conduction. Personsskilled in the art are aware that a thyristor can be turned on without agate pulse by allowing its anode voltage to increase to a predeterminedbreakover level (V or at a rate (dv/dt) in excess of a certain criticalvalue. If the dv/dt duty imposed on a thyristor exceeds this criticalrate, there is a danger of premature refiring and consequent failure ofthe inverter mechan'mm.

The snubber circuit is designed to limit the maximum rate of change ofvoltage across the valve whenever reverse recovery current terminatestherein or a stepped forward voltage is applied thereto. The snubbercapacitor and resistor should be coordinated with the external powercircuit inductance to provide a slightly underdarnped series RLCcircuit. There are a number of interrelated and sometimes competingfactors that affect the choice of values of snubber resistance andcapacitance, the principal tradeoffs being dv/dl suppression, damping,and efiiciency. For example, low resistance is desirable for the sake oflow losses and to reduce the steepness of the initial dv/dt imposed onthe valve. 0n the other hand, higher resistance helps to limit theinitial inrush of current contributed to the valve by the discharge ofthe snubber capacitor when the valve turns on, and it also helps todampen oscillations and thereby limit anode voltage overshoot followingturn off of the valve.

One objective of the present invention is to provide improved dampingmeans, useful in combination with thyristors which are protected byprior art snubber circuits, for con trolling anode voltage overshootwith relatively low initial cost and operating losses.

I have found that in some conversion apparatus the series RLC circuitsin an inactive section of the converter, which circuits are formed bythe interaction of the power circuit inductance and the snubberresistance and capacitance across turned-off valves of the inactivesection, can become oscillatory on the occasion of step changes in thevoltage thereacross due to switching in another section of theconverter. Such oscillations or ringing can cause overshoot of anodevoltage on a turned off valve. Anode voltage overshoot is undesirablebecause it tends to overstress the snubber resistors, and if the anodevoltage rises too fast or too high there is a risk of improper firing ofthe valve. Accordingly, it is another objective of my invention toprovide improved means for damping such oscillations without introducingappreciably higher losses in the apparatus.

A general object of the invention is to provide an improved dampingcircuit which is useful in a variety of solid-state switching circuitsthat are subject to relatively abrupt changes of voltage.

In carrying out my invention in one form, a capacitive reactanceelement, which is part of a snubber circuit for a thyristor, is seriallyconnected with an inductive reactance element in a circuit which issubject to relatively abrupt changes of voltage, and oscillation in thecircuit is damped by connecting across one of the aforesaid reactanceelements the series combination of a resistance, an inductance, and acapacitance. The ohmic value of the resistance is selected to damposcillations in the circuit on the occasion of the aforesaid abruptvoltage change, and the inductance and capacitance are tuned toapproximately the natural frequency of the circuit. As a result of thistuning, the resistance can have a relatively low power rating, and itdoes not add appreciable losses to the circuit except during saidoscillations.

My invention will be better understood and its various objects andadvantages 'will be more fully appreciated from the followingdescription taken in conjunction with the accompanying drawing in which:

FIG. 1 is a schematic diagram of two three-phase, doubleway, six-pulsebridges interconnected to form a reversing rectifier or cycloconvertertype of conversion apparatus with which my damping means isadvantageously used in practice;

FIG. 2 is an equivalent circuit diagram of an oscillatory circuit thatexists in the FIG. 1 apparatus; and

FIG. 3 is a graph useful for selecting the ohmic value of the resistanceneeded in my damping means to yield a predetermined overshoot factor,given the damping factor of the equivalent circuit.

For purposes of illustrating the presently preferred embodiment of myinvention, I have shown in FIG. 1 two duplicate bridges which are partof electric power conversion apparatus that controls the interchange ofpower between a source 11 and a load 12. These bridges comprise,respectively, the forward section and the reverse section of a wholeconverter. As shown, each bridge comprises a group of six identicalelectric valves connected in a conventional three-phase, doubleway,six-pulse configuration between three a-c terminals and a pair of d-cterminals. The same valves could be alternatively arranged in otherknown configurations, such as six-phase, single-way, six-pulse bridges.Preferably both bridges share the same a-c terminals a, b, and c whichserve as input terminals for the illustrated converter, and accordinglythe source 11 is connected to these terminals for supplying them withthree-phase sinusoidal voltage which alternates at a fundamentalfrequency such as 60 hertz. Usually the source is coupled to theconversion apparatus by means of power transformer windings (not shown),and it is common practice to provide overcurrent protective means forsuppressing the firing of the valves, thereby turning off the converter,in response to excessively high current in the source connections.

The d-c terminals of the illustrated bridges serve as the converteroutput terminals. As is shown in FlG. l, the forward and the reversebridges of the converter have separate sets of output terminals whichare connected in oppositely poled relationship to the load 12. A firstoutput terminals pF of the forward bridge has a positive polarity and isconnected directly to one side D of the load circuit, while the otheroutput terminal nF of the same bridge is negative and is connected via acurrent-limiting inductive reactance element 13 to the other side E ofthe load circuit. On the other hand, the first or positive outputterminal pR of the reverse bridge is connected directly to the latterside E of the load circuit, and the second or negative output terminalnR of this bridge is connected via another current-limiting inductivereactance element 14 to the one side B. Thus current can flow throughthe load circuit in a predetermined positive or forward direction whenthe forward bridge is active or in a negative or reverse direction whenthe reverse bridge is active. The load 12 may comprise the armature of avariable speed, reversible d-c motor, or it may include one or morewindings of a variable speed, multi-winding a-c motor.

In normal operation the two bridges of the converter are al' ternativelyactive. In the abnormal event of simultaneous conduction by valves inboth bridges, the inductors l3 and 14 which are connected between thebridges will limit short circuit current and thereby prevent seriousdamage to the valves while the converter is being turned off by theovercurrent protective means.

The six valves of the forward bridge have been identified by thereference 1F through 6F, respectively. The cathodes of the odd-numberedvalves are connected in common to the positive terminal pF of thisbridge, and the anodes of the even numbered valves are connected incommon to the negative terminal nF. The anode of valve 1F and thecathode of valve 4F are both connected via the first a-c terminal a ofthe converter to phase A of the alternating voltage source 11. The anodeof valve 3F and the cathode of valve 6F are both connected via a secondac terminal b to phase B of the source, and the anode of valve F and thecathode of valve 2F are both connected via the third a-c terminal 0 tothe third phase V of the source.

While each of the valves in the forward bridge is illustrated by asingle symbol, in practice it often comprises a plurality of separatethyristors connected in series and/or in parallel with one another andsuitably arranged to operate in unison. Where thyristors are paralleled,it is customary to connect current balancing inductors in seriestherewith, and in FIG. 1 such inductors are shown symbolically at 15.For very high current loads, extra bridges can be provided, with theiroutput terminals respectively coupled to terminals 16d and 16e so as tosupply additional unidirectional current to the connected load. If theinput voltages and the valve firings of a second forward bridge arephase displaced 30 electrical degrees with respect to the correspondingquantities of the illustrated bridge, a 12-pulse converter is formed, inwhich case the inductors 13 will perform the function of an interphasereactor.

The conversion apparatus includes control means for periodicallyswitching the respective valves of the forward bridge from blockingstates to forward current conducting states. Toward this end, anappropriately timed family of discrete gate pulses is cyclicallygenerated and sequentially applied to the respective control electrodes(gates) of the valves lF-6F, whereby the valves are triggered innumerical sequence in synchronism with the alternating voltage of thesource (a conventional phase rotation A-B-C is assumed), and the flow ofpower through the forward bridge is controlled as desired. Firingcircuits 17 of suitable design are used for generating and distributingthe requisite gate pulses and for determining their characteristicfiring angle.

The six valves of the reverse bridge have been identified by thereference characters 1R through 6R, respectively, and they areinterconnected and arranged similarly to the valves of the companionforward bridge. The firing circuits 18 associated with the reversebridge are similar in function and operation to the forward firingcircuits 17.

Master control means 19 is provided for alternatively activating the twofiring circuits 17 and 18 and for varying their respective firing anglesas desired. The average magnitude of voltage applied to the load circuitcan be reduced from maximum to zero by retarding the firing angle from 0toward It will be understood that the master control block 19 representssuitable control, regulation, and restraint circuitry to command theproper firing circuits 17 or l8 to initiate conduction of the valves ofthe associated bridge at the proper firing angle to vary the directionand magnitude of current delivered to the load 12 according to a presetor manual program. Once turned on, each valve in turn will continueconducting until forward current therein is decreased to zero by acyclic commutation process, and each conducting period is immediatelyfollowed by an interval of reverse anode-tocathode voltage across thatvalve. It is common practice in the relevant art to provide interlockmeans 20 so that the operation of the forward firing circuits 17 isalways inhibited or suppressed whenever reverse current is beingconducted by the reverse bridge and so that operation of the reversefiring circuits 18 is always inhibited or suppressed whenever forwardcurrent is being conducted by the forward bridge. More information aboutthe function of the interlock means and a useful embodiment thereof canbe obtained from copending patent application Ser. No. 836,765 filed onJune 26, 1969, for G.R. Lezan and myself and assigned to the GeneralElectric Company.

As can be seen in FIG. 1, each valve in the conversion apparatus isshunted by a snubber circuit comprising a resistor 21 in series with acapacitive reactance element 22. The purpose of the snubber circuit wasexplained in the introductory portion of this specification, and it iswell understood by persons skilled in the art. Where thyristors areseriesed to form a valve, each is shunted by an individual snubbercircuit, in which case these circuits will form an R-C bypass networkwhich serves the additional beneficial purpose of forcing transient andsteady-state voltage sharing among the seriesed thyristors.

During operation of the illustrated converter, the respective valves ofa bridge are fired in numerical sequence as previously explained. Assumethat the forward section is active and that the valves 1F and 2F areconducting load current. In this recurrent state of the switching means,the output terminal pF will be energized in accordance with theelectrical potential of the first input temtinal a (i.e., phase A of thesource 1 1). Valve 3F is the next one to operate, and after it has beenfired the output terminal pF is energized in accordance with theelectric potential of the input terminal b (phase B of the source 11).If the firing angle is very retarded, this change in state of theswitching means will occur at a time when the magnitude of the phase Bvoltage is appreciably higher than that of the phase A voltage. As aresult, it is accompanied by a relatively abrupt change of the voltageacross the inactive inductor 14 in series with the respectiveeven-numbered valves in the reverse section of the inverter.

In particular, the voltage across the series combination of the inductor14 and the snubber circuit 21, 22 shunting the valve 4R, whichcombination is connected between terminals pF and a, is subject tostepped increases equal to the difference between the phase B voltageand the phase A voltage at the time commutation occurs from valve 1F tovalve 3F. Assuming that valve 3F was turned on at a retarded firingangle, the voltage increase can be relatively great and commutation willbe completed rapidly, and there is consequently a tendency for thisseries RLC combination to be oscillatory or to ring and for the voltageacross the thyristors in the valve 4R to rise to an even highermagnitude. Anode voltage overshoots of as high as 60 percent have beenobserved in practice. This undesirably stresses and weakens theresistors 21 in the snubber circuits. Where each valve comprises two ormore thyristors in series and the snubber resistor across one of thethyristors fails (becomes open circuited), all of the voltage will thenbe impressed on that one thyristor which may then turn on by V or dv/dtaction, whereupon the remaining thyristor is similarly fired. As aresult of simultaneous conduction by valves in the forward and reversebridges, the source 11 is short circuited and the overcurrent responsiveprotective means will suppress all valve firings, thereby shutting downthe converter. This deenergizes the load 12 at a time that may beharmful to the machinery or process being driven thereby.

In accordance with my invention, the voltage overshoot problem reviewedin the preceding paragraph is solved by shunting either the inductor 14or the snubber circuit 21, 22 with a tuned damping circuit 23characterized by resistance, inductance, and capacitance in series withone another. The resistance has an ohmic value selected to damposcillations in the above-mentioned series combination on the occasionof a stepped voltage increase, and the inductance and capacitance aretuned to approximately the natural frequency of the series combination.Additional information regarding the selection of parameters will be setforth soon hereinafter. In the illustrated converter, the tuned dampingcircuit 23 is shown connected in parallel with inductor 14. There is aduplicate circuit 23 in parallel with the inductor 13 to dampenoscillations in the forward bridge while the reverse bridge is active.As indicated in FIG. 1, each of the damping circuits 23 comprises aresistor 24 of R ohms in series with an inductor 25 of L henries inseries with a capacitor 26 of C farads. For a better understanding ofthe criteria for selecting these parameters, FIGS. 2 and 3 have beenincluded in the drawing, and they will now be described.

In FIG. 2 an equivalent circuit of the FIG. 1 converter is shown. Theswitch 31 is equivalent to the controllable switching means or valves ofthe forward section of the converter, and the closure of this switchcorresponds, for exampie, to the firing of valve 3F and rapidcommutation from phase A to phase B of the source 11 in FIG. 1. Thebattery 32 in FIG. 2 is a source of voltage (V,,) equal to the voltagebetween phase B and phase A when the switch 31 is closed. The equivalentinductance L equals the sum of the inductances of the current limitingreactive element 14 and the balancing reactors 15. (As a firstapproximation, I... can be considered the same as L,, which is theinductance of the current-limiting inductor and is usually much largerthan that of the balancing reactors.) The thyristor 33 represents aturnedoff valve (e.g., 4R) in the reverse section of the converter, andV is the output voltage thereacross. Capacitance of C farads isconnected in parallel with thyristor 33, and resistance of R, ohms isconnected across the thyristor in series with this capacitance. R and C,are equivalent, respectively, to all resistors and capacitors in thesnubber circuits associated with the even-numbered valves of the reversebridge. In practice the equivalent circuit thus far described may have adamping that is relatively low (i.e., less than 0.25).

The switch 31 in FIG. 2 is periodically operative in a manner thatsubjects the series combination of L, and C, to a relatively abruptvoltage increase. To damp oscillations in the series combination on thisoccasion, the resistance R is connected across the inductance L Thedegree of damping depends on the ohmic value of R By selecting anappropriate value for R the overshoot factor of the circuit can bereduced to an acceptable level. By overshoot factor I mean the maximumratio of the output voltage across the thyristor (V to the drivingvoltage (V,,).

Given the damping factor of the original circuit and the maximumovershoot that is permissible, persons skilled in the art will know howto select the proper value of R For added convenience, I have shown inFIG. 3 a family of curves representing overshoot factor vs. the quantityi E R 2 C,

for a variety of damping factors that are typical of the equivalentcircuit shown in FIG. 2. It should be noted that in plotting thesecurves I have ignored the equivalent resistance R, which is trivial ifthe damping factor is low.

I-Iaving selected the value of R I reduce losses in the damping circuitwithout appreciable increasing the over-shoot factor by connecting inseries therewith the inductance L and capacitance C: which are tuned tothe natural frequency of the original circuit. In other words, 0 equalsapproximately L C As a practical matter, significantly improved resultswill be obtained even if the ratio of these products were greater orless than one (e.g., between 1.33 and 0.75 The utility of the tuneddamping circuit is improved by keeping C as low as possible, consistentwith the following two restraints.

The first of these restraints is imposed to ensure that the tuneddamping circuit itself is reasonably well damped. In the secondrestraint the fraction is the damping factor of the loop comprising thetuned damping circuit in combination with the output circuit R,C,; theeconomics of my invention becomes unfavorable if this damping factor ishigher than 1.

As was previously mentioned, my tuned damping circuit can be connectedalternatively in parallel with the snubber circuit. This possibility,which is illustrated by broken lines in FIG. 2, may be advantageous inforced commutation inverters or for any case where the main inductance Lis inaccessible.

When connected in this manner, my invention will beneficially augmentthe conventional damping function of the snubber capacitor on theoccasion of turning ofi the thyristor 33.

While I have shown and described a preferred form of the invention byway of illustration, many modifications will probably occur to thoseskilled in the art. I therefore contemplate by the claims which concludethis specification to cover all such modifications as fall within thetrue spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In an electric power circuit comprising a thyristor, a capacitivereactance element connected in parallel with said thyristor, aninductive reactance element connected in series with the parallelcombination of said thyristor and said capacitive reactance element, aplurality of input terminals adapted to be connected to a source ofvoltage, and means including controllable switching means forelectrically interconnecting said input terminals and the seriescombination of said capacitive reactance element and said inductivereactance element, said switching means being periodically operative ina manner that subjects said series combination to a relatively abruptchange of voltage, the improvement comprising:

a tuned damping circuit connected in parallel with a predetermined oneof said reactance elements, said damping circuit being characterized byresistance, inductance, and capacitance in series with one another, saidresistance having an ohmic value selected to damp oscillations in saidseries combination on the occasion of said abrupt voltage change, andsaid inductance and said capacitance being tuned to approximately thenatural frequency of said series combination.

2. The circuit of claim 1 in which said predetermined one reactanceelement is said inductive reactance element.

3. The circuit of claim 1 in which a resistor is connected across saidthyristor in series with said capacitive reactance element.

4. In an electric power converter comprising a thyristor, a capacitivereactance element connected in parallel with said thyristor, aninductive reactance element connected in series with the parallelcombination of said thyristor and said capacitive reactance element, aplurality of input terminals adapted to be connected to a source ofvoltage, a set of at least first and second output terminals, meansincluding controllable switching means for electrically interconnectingsaid input and output terminals, said switching means having onerecurrent state in which said first output terminal is energized inaccordance with the electrical potential of a first one of said inputterminals and being periodically operative to another state in whichsaid first output terminal is energized in accordance with theelectrical potential of another one of said input terminals, the seriescombination of said capacitive reactance element and said inductivereactance element being connected between said first output terminal andsaid first input terminal, whereby upon operation of said switchingmeans the voltage across said series combination is subject to arelatively abrupt change, the improvement comprising:

a tuned damping circuit connected in parallel with a predetermined oneof said reactance elements, said damping circuit being characterized byresistance, inductance, and capacitance in series with one another,

said resistance having an ohmic value selected to damp oscillations insaid series combination on the occasion of said abrupt voltage change,and said inductance and said capacitance being tuned to approximatelythe natural frequency of said series combination.

5. The converter of claim 4 in which means is provided for connectingsaid first output terminal directly to one side of an electric powerload circuit and for connecting said second output terminal via acurrent limiting inductor to the other side of the load circuit.

6. The converter of claim 4 in which said input terminals are adapted tobe connected to a source of 3-phase alternating voltage, means isprovided for directly connecting said first output terminal to one sideof an electric power load circuit and for connecting said second outputterminal to the other side of the load circuit, and said controllableswitching means comprises a group of six periodically conductingelectric valves connected in a first bridge configuration between saidinput and output terminals, said valves being cyclically fired, when thefirst bridge is active, in a predetermined sequence so as to supplyunidirectional current to the connected load circuit.

7. The subject matter of claim 6 wherein the converter comprises twogroups of valves forming first and second duplicate bridges which sharethe same input terminals but have separate sets of output terminals,said bridges being alternatively active and their respective sets ofoutput terminals being connected in oppositely poled relationship tosaid load circuit so that current can flow through said load circuit ina predetermined forward direction when the first bridge is active or ina reverse direction when said second bridge is active, each valve ofsaid second bridge comprising at least one thyristor which is shunted bya capacitive reactance element; and said inductive reactance element isconnected between the second output terminal of said second bridge andthe first output terminal of said first bridge.

8. The converter of claim 7 in which said predetermined reactanceelement is said inductive reactance element.

9. The converter of claim 8 in which the first output terminal of saidsecond bridge is directly connected to said other side of the loadcircuit, each valve of said first bridge comprises at least onethyristor which is shunted by a capacitive reactance element, anadditional inductive reactance element is connected between the secondoutput terminal of said first bridge and the first output terminal ofsaid second bridge, and said additional inductive reactance element isshunted by another tuned damping circuit which is a duplicate of thetuned damping circuit specified in claim 1.

1. In an electric power circuit comprising a thyristor, a capacitivereactance element connected in parallel with said thyristor, aninductive reactance element connected in series with the parallelcombination of said thyristor and said capacitive reactance element, aplurality of input terminals adapted to be connected to a source ofvoltage, and means including controllable switching means forelectrically interconnecting said input terminals and the seriescombination of said capacitive reactance element and said inductivereactance element, said switching means being periodically operative ina manner that subjects said series combination to a relatively abruptchange of voltage, the improvement comprising: a tuned damping circuitconnected in parallel with a predetermined one of said reactanceelements, said damping circuit being characterized by resistance,inductance, and capacitance in series with one another, said resistancehaving an ohmic value selected to damp oscillations in said seriescombination on the occasion of said abrupt voltage change, and saidinductance and said capacitance being tuned to approximately the naturalfrequency of said series combination.
 2. The circuit of claim 1 in whichsaid predetermined one reactance element is said inductive reactanceelement.
 3. The circuit of claim 1 in which a resistor is connectedacross said thyristor in series with said capacitive reactance element.4. In an electric power converter comprising a thyristor, a capacitivereactance element connected in parallel with said thyristor, aninductive reactance element connected in series with the parallelcombination of said thyristor and said capacitive reactance element, aplurality of input terminals adapted to be connected to a source ofvoltage, a set of at least first and second output terminals, meansincluding controllable switching means for electrically interconnectingsaid input and output terminals, said switching means having onerecurrent state in which said first output terminal is energized inaccordance with the electrical potential of a first one of said inputterminals and being periodically operative to another state in whichsaid first output terminal is energized in accordance with theelectrical potential of another one of said input terminals, the seriescombination of said capacitive reactance element and said inductivereactance element being connected between said first output terminal andsaid first input terminal, whereby upon operation of said switchingmeans the voltage across said series combination is subject to arelatively abrupt change, the improvement comprising: a tuned dampingcircuit connected in parallel with a predetermined one of said reactanceelements, said damping circuit being characterized by resistance,inductance, and capacitance in series with one another, said resistancehaving an ohmic value selected to damp oscillations in said seriescombination on the occasion of said abrupt voltage change, and saidinductance and said capacitance being tuned to approximately the naturalfrequency of said series combination.
 5. The converter of claim 4 inwhich means is provided for connecting said first output terminaldirectly to one side of an electric power load circuit and forconnecting said second output terminal via a current limiting inductorto the other side of the load circuit.
 6. The converter of claim 4 inwhich said input terminals are adapted to be connected to a source of3-phase alternating voltage, means is provided for directly connectingsaid first output terminal to one side of an electric power load circuitand for connecting said second output terminal to the other side of theload circuit, and said controllable switching means comprises a group ofsix periodically conducting electric valves connected in a first bridgeconfiguration between said input and output terminals, said valves beingcyclically fired, when the first bridge is active, in a predeterminedsequence so as to supply unidirectional current to the connected loadcircuit.
 7. The subject matter of claim 6 wherein the convertercomprises two groups of valves forming first and second duplicatebridges which share the same input terminals but have separate sets ofoutput terminals, said bridges being alternatively active and theirrespective sets of output terminals being connected in oppositely poledrelationship to said load circuit so that current can flow through saidload circuit in a predetermined forward direction when the first bridgeis active or in a reverse direction when said second bridge is active,each valve of said second bridge comprising at least one thyristor whichis shunted by a capacitive reactance element; and said inductivereactance element is connected between the second output terminal ofsaid second bridge and the first output terminal of said first bridge.8. The converter of claim 7 in which said predetermined reactanceelement is said inductive reactance element.
 9. The converter of claim 8in which the first output terminal of said second bridge is directlyconnected to said other side of the load circuit, each valve of saidfirst bridge comprises at least one thyristor which is shunted by acapacitive reactance element, an additional inductive reactance elementis connected between the second output terminal of said first bridge andthe first output terminal of said second bridge, and said additionalinductive reactance element is shunted by another tuned damping circuitwhich is a duplicate of the tuned damping circuit specified in claim 1.